The invention relates to a folder for use in the trans¬ port of an original sheet and a copy sheet, engaged with each other, past a heat radiator in a ther o- copying machine, said folder comprising at least one flexible carrier sheet and one likewise flexible cover sheet which is permeable to heat rays. Such a folder serves to keep the original sheet and the copy sheet together and also to protect the copying material against heating or cooling ^" by convection heat which has a fogging effect because it reduces the contrast between the parts to be heated and those which are not to be heated. In some cases, the use of a folder is also necessary or the sheet material which is moved through the thermocopying machine to become sufficient¬ ly rigid to actuate electric switch means fitted along the path of movement of the sheet material.

Besides having optimum permeability to infra-red rays, the cover sheet, which is present between the copying material and the heat radiator during the copying process, must also be resistant to heat and mechanical influences and be inexpensive to produce. Silk and nylon gauze are used to some extent owing to their mechanical strength and good permeability to heat rays, but these materials are rather delicate as mote or other dirt particles which settle on such a cover sheet easily burn on to the sheet or cause the sheet to be burned through.

Frequently used materials are smooth plastics foils of e.g. PTFE (polyte ^' trafluoroethylene) or FΞP (fluor- inated ethylenepropylene copoly erisate) , both of which

absorb little radiant heat and endure rather high temperatures. A drawback of them and other smooth foils is, however, that they engage the copying material over a large area and are thus in a good heat transfer contact with it so that the copying material is not only heated locally by the infra-red rays, but also receives an evenly distributed heat which is transferred by convection.

The object of the invention is to provide a folder of the present type which in addition to having the resistance of the good plastics materials to thermal and mechanical influences, offers poor conditions for heat transfer by convection.

This object is achieved in that the cover sheet is ^" made by a subsequent treatment of smooth plastics foil to produce a predetermined pattern of depressions in and raised portions on one or both surfaces, as stated in claim 1, said arrangement ^' reducing the area of contact with the copying material and thus the flow of heat that can be transferred by convection through said area.

The subsequent treatment may e.g. comprise the cutting, pressing or scratching of dense grooves in the sheet by means of a suitable tool which causes the area of contact with the copying material to be resolved in linear or narrow strips. An even better effect can be achieved by producing two sets of intersecting grooves so that the contact face is resolved in dots or small spots. Such working may e.g. be effected by diamond knurling.

It has been found that the use of a diamond-knurled

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PTFA cover sheet increases the speed at which a set of copying material can be moved through a given thermocopy¬ ing machine by approximately 2 % compared to a corre¬ sponding smooth cover sheet, and that the copying quality is improved at the same time. This reduc¬ tion in transport time also causes the machine to be heated less.

In addition to the carrier sheet. and the cover sheet, the folder may also be provided, in a known manner, with a pad of resilient material, such as foam rubber, disposed on the carrier sheet and with a heat reflect¬ ing sheet, e.g. of aluminium foil, disposed between said pad and the cover sheet.

However, it may also be desired to reduce the transfer of heat by convection to and from the carrier sheet, and it may therefore be expedient that this sheet, too, has a pattern of depressions and raised portion produced by a subsequent treatment, as stated in claim 2.

The invention will be explained more fully below with reference to the drawing, in which

fig. 1 diagra matically shows the most essential parts of an ordinary thermocopying machine with a folder comprising a set of copying material being moved through the apparatus, and

fig. 2 is a greatly enlarged view of a part of a section through an embodiment of the folder of the invention and the set of copying material placed in it.

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In the drawing 10 represents a cylindrical glass tube which is rotatably mounted between a rotatably jour- nalled rubber roller 11 and two pairs of likewise rotatable support rollers 12, one pair of which may be driven by a motor not shown. The rollers of each pair are located at their respective ends of the glass tube 10, and only one of them is shown in the drawing. Within the glass tube there is fitted a rod-shaped, stationary heat radiator comprising a heating element 13 and a reflector 14. The pins 15 of the rubber rol¬ ler 11 are subjected to the action of springs 16 which keep the roller in resilient engagement with the glass tube 10.

The folder and the copying material included in it are as a whole designated by 20. For clarity, the va¬ rious sheets in the drawing are shown drawn somewhat apart though in reality they are disposed closely ad¬ jacent one another. As appears from fig. 2, the fol¬ der comprises a flexible carrier sheet 21 and a like- wise flexible cover sheet 22 which are adhered to each other along one edge in a suitable, not shown manner.

The carrier sheet 21 is flexible, as mentioned, but does have such a degree of rigidity that it will not crack or be folded during normal handling. Preferably, it consists of plastics, e.g. polyester or nylon, but may also consist of other materials, such as card¬ board or strong paper.

The cover sheet 22 is permeable to heat rays and pre- ferably consists of one of the plastics types PTFE and FEP mentioned in the foregoing. It is made by knurling or diamond knurling of smooth plastics foil

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so as to produce depressions 24 and raised portions 23,

The copying material consists of an original sheet 25 and a copy sheet 2β which is permeable to heat rays and its side facing the original sheet has a heat sensitive layer that changes its colour on heating. Such heating is effected at the locations where the original sheet is dark and therefore absorbs the heat rays when the locations in question are past the heat radiator 13 _? 14 during the transport of the material between the glass tube 10 and the rubber roller 11.

The irregular surface of the cover sheet 22 causes the cover sheet to come in convection heat trans- ferring contact with the glass tube 10 only on the tops of the raised portions 23 and with the copy sheet 26 at the bottom of the depressions 24 which greatly reduces the fogging heating, which takes place as a consequence of the convection heat.

When the original sheet is transparent it is usually expedient to fit the original sheet and the copy sheet in the reverse order of that shown, i.e. with the original sheet closest to the heat radiator.

The rubber roller is gradually heated when it has been in use for an extended period of time so heat may be transferred from it to the copy sheet by con¬ vection. To reduce this transfer of heat an irregu¬ lar surface may be imparted to the carrier sheet 21, too, in the same manner as the cover sheet 22.

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